Integration of Thermoelectric Conversion with Reverse Electrodialysis for Mitigating Ion Concentration Polarization and Achieving Enhanced Output Power Density

Concentration polarization (CP) is a ubiquitous phenomenon in membrane separation that is possibly responsible for the decline in overall efficiency. However, no quantitative connection between the key membrane parameters and the degree of CP has been established. In this study, the variation in permselectivity during CP was comprehensively elucidated using isostructural covalent organic framework (COF) membranes with varied charge densities. A volcano-like plot of output power density against charge population was obtained, in contrast to the conventional presumptions regarding an increase in the membrane charge density leading to an enhancement in permselectivity, and a consequent increase in osmotic voltage. The CP-induced detrimental effects were considerably mitigated by imposing a temperature gradient on the solutions, which led to an increase in the thermophoretic mobility of ions. The reverse electrodialysis (RED) device with alleviated CP yielded an output power density of up to 24.7 mW cm –2 with a temperature difference of ∼40 K; this value far exceeds the commercial benchmark, thus establishing a new criterion for RED batteries. These results outline a previously undiscovered benefit for linking thermoelectric conversion and RED to realize advanced energy-conversion technologies.